IGCSE Biology: inheritance and genetic crosses

The definitions examiners expect

What each board covers

Cambridge 0610 Core students define the terms above, describe sex inheritance with X and Y chromosomes, interpret pedigree diagrams, and use genetic diagrams and Punnett squares for monohybrid crosses giving 1 : 1 and 3 : 1 ratios. In the 2026 to 2028 syllabus, mitosis, meiosis, haploid and diploid, protein synthesis via mRNA, test crosses, codominance, ABO blood groups and sex linkage (red-green colour blindness) are all Supplement content, so Extended candidates must know them.

Edexcel 4BI1 covers alleles, the same key terms, monohybrid genetic diagrams, family pedigrees, probabilities of outcomes and sex determination for everyone, plus mitosis and meiosis (3.28 to 3.32), polygenic inheritance (3.22) and mutation. Codominance (3.21B), DNA structure (3.16B), RNA (3.17B) and transcription and translation (3.18B) are examined only on Paper 2. Edexcel does not name ABO blood groups, test crosses or sex linkage, though a question can still give you unfamiliar information to apply.

Worked example 1: Tt x Tt (Punnett square)

Pea plants: T = tall (dominant), t = short (recessive). Both parents are heterozygous. Gametes from each parent: T or t.

Genotypes 1 TT : 2 Tt : 1 tt. Phenotypes 3 tall : 1 short. Probability of a short plant = 1 in 4, or 0.25, or 25%.

Worked examples 2 and 3: test cross and sex linkage

Test cross (Cambridge Supplement): a tall plant could be TT or Tt. Cross it with a short (tt) plant. If all offspring are tall, the unknown parent was almost certainly TT, because it can only give T gametes. If about half the offspring are short, the parent was Tt, since Tt x tt gives Tt and tt in a 1 : 1 ratio. Always say why the homozygous recessive is used: it can only pass on recessive alleles, so the offspring's phenotypes reveal the unknown parent's alleles.

Sex linkage (Cambridge Supplement): the allele for red-green colour blindness (Xb) is recessive and carried on the X chromosome. A carrier mother (XB Xb) and an unaffected father (XB Y). Mother's gametes: XB or Xb. Father's gametes: XB or Y. Offspring: XB XB (unaffected girl), XB Xb (carrier girl), XB Y (unaffected boy), Xb Y (colour-blind boy). So 1 in 4 of all children is expected to be colour blind, which is 1 in 2 of the sons, and no daughters. This is why the condition is more common in males: they have only one X chromosome.

ABO blood groups (Cambridge Supplement): IA and IB are codominant, Io is recessive. A parent with genotype IA Io crossed with IB Io can produce IA IB (group AB), IA Io (A), IB Io (B) and Io Io (O) in a 1 : 1 : 1 : 1 ratio.

Common mistakes

• Skipping the gametes line. Mark schemes give a mark for parental genotypes, one for gametes, one for offspring genotypes and one for phenotypes linked to them.
• Writing a ratio as a probability without saying which: 3 : 1 is a ratio, 0.75 is a probability. Read which one the question asks for.
• Using different letters for the two alleles (T and s). Use one letter, capital for dominant, lower case for recessive, and choose letters whose cases look different.
• Thinking a 3 : 1 ratio means exactly 3 of every 4 offspring will be tall. It is an expected ratio; real numbers vary by chance.
• Saying the father's sperm carries an X or Y "chromosome pair". Gametes carry one sex chromosome.
• Confusing genes with alleles, or chromosomes with genes.

Exam technique for genetics questions

Set out every cross the same way, with labels: parental phenotypes, parental genotypes, gametes (circled is a common convention), offspring genotypes (Punnett square), offspring phenotypes, ratio or probability. Even when the question only asks for the ratio, laying out the diagram earns method marks if the final answer slips.

For pedigree diagrams, find two unaffected parents who have an affected child: that proves the condition is recessive, and both parents must be heterozygous. State that reasoning in words; the examiner needs the evidence, not just the conclusion. Edexcel also expects you to explain why most features, such as height in humans, are polygenic and show continuous variation.

How one-to-one lessons help

Genetics is pattern work, and most students improve fast once a tutor makes them lay out every cross the same way. In lessons the tutor builds crosses on the shared whiteboard with the student, then hands over unseen pedigree and probability questions and checks each line of working against the mark scheme. For Extended or Edexcel Paper 2 candidates, the tutor adds sex linkage, codominance or protein synthesis depending on the board.

Self-check

1. Define gene, allele, genotype, phenotype, homozygous, heterozygous, dominant and recessive.
2. Draw a full genetic diagram for Tt x Tt and Tt x tt.
3. Convert a ratio into a probability and a percentage.
4. Use a pedigree to show a condition is recessive.
5. Show how sex is determined at fertilisation.
6. (Cambridge Supplement) Explain a test cross and predict ABO and colour blindness crosses.
7. (Edexcel Paper 2) Describe transcription and translation in outline.